This invention relates to an amphibious device for monitoring nuclear fuel element growth as a consequence of irradiation and more particularly deals with an amphibious end plug gauging device for subaqueously measuring the distance between fuel element end plugs and the upper tie plate surface of a fuel bundle comprising the fuel elements.
Fuel bundles in a reactor core include a plurality of fuel elements vertically positioned between upper and lower tie plates, and horizontally secured by spacers positioned at predetermined intervals between the tie plates. When the bundles are removed from the core after reactor operation, they are "hot" or irradiated and are typically stored underwater in a reactor service pool for radiation shielding. The fuel elements between the tie plates are individually capped at each end, to prevent the escape of fission products and fission gas. This is accomplished by welding so called "end plugs" including cylindrical shanks onto the ends of the elements. Some of the shanks are threaded, permitting the corresponding fuel elements to be screwed into place or held down by nuts on the shank. In most instances, however, the shanks are smooth and the fuel elements are merely inserted into place by fitting the shanks into receiving shank holes drilled through the tie plates. Each fuel element is provided with a downwardly biasing spring bearing against the underside of the upper tie plate and concentrically mounted about the shank of the upper end plug.
During reactor operation, radiation causes the fuel elements between the upper and lower tie plates to grow at varying rates. The elements secured onto the tie plates may grow more slowly or more rapidly than those merely inserted into shank holes of the tie plates--possibly causing separation between the upper tie plate and a number of the fuel elements, or conversely possibly causing certain fuel elements to bow laterally due to insufficient room for axial expansion. In either case, whether separated from the upper tie plate and thereby subject to vibration caused by reactor water being circulated through the fuel bundle, or bowed by excessive growth, detrimental stresses are imposed on the cladding of the fuel elements, possibly causing the cladding to rupture.
Further information on the structure of fuel elements and fuel bundles is found in U.S. Pat. No. 3,431,170, which is expressly incorporated into this disclosure and made a part thereof.
To minimize the aforementioned risks of separation and bowing of fuel elements within a bundle, regular monitoring of fuel element growth is desirable. This can be accomplished by regularly measuring the distance between end plugs and a reference or datum surface of the upper tie plate in accordance with the invention disclosed herein.
Accordingly, an object of this invention is to promote the integrity of nuclear fuel bundles, and more particularly to maintain fuel elements suitably positioned or secured within the upper tie plate of a fuel bundle.
Another object of the present invention is to provide an amphibious device capable of measuring fuel rod growth relative to an upper tie plate reference surface in a manner obviating disassembly of the fuel bundle prior to measurement.
Another object of the present invention is to promote the convenient and economical subaqueous measurement of the degree of insertion of reactor fuel elements in an upper tie plate intended for holding the upper ends of the fuel elements in a spaced relationship to one another.